1. Field of the Invention
The present invention relates to power control in a wireless communication system.
2. Description of Related Art
It is well known that power control is critical for CDMA (code division multiple access) wireless systems such as those based on the IS-95 standard (e.g., see Holtzman, J. M., xe2x80x9cCDMA Power Control for Wireless Networks,xe2x80x9d in Third Generation Wireless Information Networks, S. Nanda and D. J. Goodman (eds), Kluwer Academic Publishers, Boston, Mass., 1992; and TIA/EIA/IS-95 Interim Standard, Mobile Stationxe2x80x94Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System, Telecommunication Industry Association, July 1993). The ultimate objective of power control in CDMA systems is to achieve a desired speech quality on a particular link at a minimum transmit power level. Without effective power control, a desired level of speech quality cannot be obtained under large system loads, and as a result, the capacity gains expected from a CDMA wireless system will not be realized. This is especially true for the reverse link (uplink) of a CDMA system (i.e., from a mobile station to a base station).
Realizing the importance of power control for the reverse link, the IS-95 standard has provided for a power control scheme known in the art as xe2x80x9cinner loop power control.xe2x80x9d In this scheme, a base-station transmits a 1-bit feedback signal to a mobile station every 1.25 milliseconds (ms). The 1-bit value of this feedback signal is representative of whether an estimate of the instantaneous bit energy to noise density ratio (Eb/N0), which is representative of the signal-to-noise ratio, of the received signal at the base station (transmitted from the mobile station) exceeds, or falls below, a target ratio EbT/N0T. Hereinafter, the bit energy to noise density ratio will be referred to as the signal-to-noise ratio.
As noted above, the ultimate objective of a power control scheme in the context of CDMA systems is to achieve a desired speech quality on a particular link at a minimum transmit power level. A simple, quantifiable, measure of the speech quality on a link is the associated frame error rate (FER) on that link. For CDMA systems based on IS-95, the desired speech quality can be said to have been achieved on a link if the FER is at or below a certain level (e.g., 1%). For a given fading environment, the FER is a function of the average Eb/N0 at the receiver. Since, as described above, inner loop power control helps maintain the receiver Eb/N0 close to the target EbT/N0T, the FER is, ultimately, determined by the target EbT/N0T. Therefore, to achieve the desired speech quality in a given fading environment, the target EbT/N0T needs to be set at a level which is appropriate for that environment. Otherwise, the transmission of one mobile station could interfere with the transmission of another mobile station such that the mobile station interfered with can not be heard by the base stationxe2x80x94essentially, reducing the capacity of the base station.
Unfortunately, there is no fixed target EbT/N0T that achieves the desired FER in all fading environments. Therefore, those in the art have developed an adaptive mechanism that adjusts the target EbT/N0T accordingly. This mechanism, referred to hereafter as xe2x80x9cReverse Outer Loop Power Controlxe2x80x9d (ROLPC) monitors the FER and changes the target EbT/N0T depending on whether the FER is below, or above, a desired threshold. By directly using the FER to drive the target EbT/N0T, the current ROLPC achieves its objective very well in reasonably steady fading environment. However, since the FER monitoring processing implicit in this technique is rather slow (with time constants on the order of a couple of seconds), its performance can deteriorate in a dynamic environment with rapidly changing fading characteristics.
As such, in order to improve the speed of the ROLPC, the commonly assigned U.S. Patent Application of Carl Weaver and Wei Peng, entitled xe2x80x9cSymbol Error Based Power Control for Mobile Telecommunication System,xe2x80x9d Ser. No. 08/346800, filed Nov. 30, 1994, describes a symbol error (SE) based technique which potentially improves the performance of ROLPC in a dynamic fading environment. This fixed SE rate (SER) target ROLPC technique, which is based on the premise that the SER and FER are strongly correlated, tries to maintain the SER close to a pre-determined fixed target SER value. Thus, after every frame the associated symbol error count is compared with the target SER and the EbT/N0T target is raised or lowered depending upon whether the symbol error count was above or below the SER target. The updated EbT/N0T target is used to generate inner loop feedback bits during the next frame.
The above-mentioned fixed SER target ROLPC technique uses a fixed SER target for the mean value of the SER. Notwithstanding the performance improvements possible with the above-mentioned fixed SER target ROLPC technique, it has been observed that the correlation between the SER and FER varies across different wireless communications environments. For a given (fixed) SER target, the FERs in different fading environments can differ by an order of magnitude. In other words, the above-mentioned fixed SER target ROLPC technique cannot maintain the FER close to the target in all fading environments. As such, in order to achieve a desired FER, different environments require different SER targets. Commonly assigned, U.S. patent application Ser. Nos. 09/052,696 and 09/052,581 of Rege, entitled xe2x80x9cAn Adaptive Symbol Error Rate Based Technique for CDMA Reverse Link Outer Loop Power Control,xe2x80x9d and xe2x80x9cA Non-Adaptive Symbol Error Count Based Algorithm for CDMA Reverse Link Outer Loop Power Control,xe2x80x9d respectively, describe techniques for achieving a desired FER under different fading conditions. Specifically, the adaptive technique describes an adaptive SER based power control scheme that uses the coefficient of variation (or standard deviation) of the symbol error count as a xe2x80x9csignaturexe2x80x9d of the environment for dynamically setting the SER target based on its estimate of the environment.
The alternative, non-adaptive symbol error count based reverse link outer loop power control technique also achieves the desired FER under a variety of fading environments. In particular, this technique sets a target for a second order statistic of the SE count. And, both methods use a comparison of the target with measured value to make changes in the EbT/N0T target; namely, an increase or decrease by a fixed amount. Consequently, achieving improved performance through these types of power control methods may take several iterations because the increase and decrease in the EbT/N0T target occur in fixed amounts.
The method according to the present invention provides for enhanced power control by adaptively adjusting an amount of change in a target signal-to-noise ratio. In a power control technique modified according to the present invention, the amount by which the target signal-to-noise ratio changes is a function of the standard deviation of the symbol error rate (SER), and more particularly, the difference between the standard deviation of the SER and a target standard deviation of the SER.
In one embodiment, adaptively changing the target signal-to-noise ratio is triggered by the updating of a frame error rate (FER). However, in another embodiment, as the standard deviation of the SER is updated, which occurs much more quickly than the updating of the FER, the target signal-to-noise ratio is changed. In both embodiments, when the communication quality indicated by the standard deviation of the SER and the target of the standard deviation of the SER differs from the communication quality indicated by the FER, the target standard deviation of the SER is updated so that the two quality indicators become more consistent.